Confocal Raman microscopy is a useful analysis technique which has been employed for many years. The basis of the technique relies on collecting Raman scattered light from a sample through a coincident or "confocal" aperture. The fundamental configuration of a confocal microscope is shown in FIG. 1. Light from a spatially small, spectrally pure source (typically a diffraction limited laser beam) is brought to a focus onto the sample through a microscope objective. The light enters the optical path of the objective by means of a beam combining element (typically a wavelength selective mirror which is generally a holographic element). The scattered Raman light is collected traveling backwards through the same objective. Because the Raman scatter comprises different wavelengths than the one to which the beam combining element is tuned, it passes through the combiner and is imaged into the confocal aperture. The light passing through the confocal aperture is then presented to a spectrograph for analysis. The net effect of this arrangement is to restrict the collection of Raman scatter to the small volume of the sample coincident with focused light source. This restriction of the sample space allows the isolation of small volumes of the sample from the surrounding material. Taking a number of successive data collections at different points allows the development of three-dimensional profiles of transparent samples.
Fiber optically coupled Raman microscopes have become popular recently. These devices deliver the laser light and collect the Raman scatter via fiber optics. For true confocal performance the laser light must be diffraction limited and the collection aperture must be approximately the same size as the diffraction limited spot on the sample. A singlemode fiber can be utilized to deliver the diffraction limited laser beam to the microscope, the difficulty is in the confocal collection aperture. Using a singlemode (SM) collection fiber would provide the necessary confocality, however the numerical aperture of SM fibers is quite low yielding poor data collection efficiencies. In addition, interference effects in SM fibers contaminates the Raman data. A better solution from the point of view of collection efficiency and interference effects would be to use a multimode (MM) collection fiber. Unfortunately the core size of the MM fiber is too large to be an effective confocal aperture.